1. Field of the Invention
The present invention relates to a workpiece carrier for holding a workpiece such as a semiconductor wafer while the workpiece is being polished to make a surface of the workpiece exhibit a flat mirror finish, and a polishing apparatus having such a workpiece carrier.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 xcexcm wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus.
Conventionally, a polishing apparatus has a turntable having a polishing cloth attached thereon, and a top ring for applying a constant pressure against the turntable. A semiconductor wafer to be polished is placed on the polishing cloth and clamped between the top ring and the turntable, and the surface of the semiconductor wafer on which circuits are formed is chemically and mechanically polished, while supplying a polishing liquid onto the polishing cloth. This process is called chemical mechanical polishing (CMP).
The polishing apparatus is required to have such performance that the surfaces of semiconductor wafers have a highly accurate flatness. Therefore, it is considered that the holding surface, i.e. the lower end surface of the top ring which holds a semiconductor wafer, and the upper surface of the polishing cloth which is held in contact with the semiconductor wafer, and hence the surface of the turntable to which the polishing cloth is attached, preferably have a highly accurate flatness, and the holding surface and the surface of the turntable which are highly accurately flat have been used. It is also considered that the lower surface of the top ring and the upper surface of the turntable are preferably parallel to each other, and such parallel surfaces have been used.
The semiconductor wafers to be polished, with circuits formed thereon, do not have a uniform thickness over their entire surfaces. There has been an attempt to attach an elastic pad made of polyurethane or the like to the holding surface of the top ring for holding a semiconductor wafer for thereby uniformizing a pressing force applied from the top ring to the semiconductor wafer to be polished, over the entire surface of the semiconductor wafer. When the pressing force applied to the semiconductor wafer is uniformized by the elastic pad, the semiconductor wafer is prevented from being polished in a localized region for thereby improving flatness of the polished surface of the semiconductor wafer.
However, the method for uniformizing the pressing force applied to the semiconductor wafer by elasticity of the elastic pad fails to meet stricter requirements for increased flatness of polished semiconductor wafers.
Efforts have also been made to employ a diaphragm made of an elastic material such as rubber as the holding surface of the top ring for holding a semiconductor wafer, and apply a fluid pressure such as an air pressure to the reverse side of the diaphragm to uniformize the pressing force applied to the semiconductor wafer over its entire surface. The top ring with the diaphragm used as its holding surface includes a guide ring or retainer ring disposed at the outer circumferential edge of the diaphragm for holding a semiconductor wafer. The outer circumferential portion of the diaphragm corresponds to the outer circumferential portion of the semiconductor wafer, and the outer circumferential edge of the diaphragm needs to be fixed to the top ring or the guide ring. Therefore, even when a fluid pressure such as an air pressure is applied to the reversed side of the diaphragm, the outer circumferential portion of the diaphragm is elastically deformed to a smaller extent than the outer area of the diaphragm, and hence tends to become a point of inflection. Accordingly, the polishing pressure applied to the outer circumferential portion of the semiconductor wafer is smaller than the polishing pressure applied to the other area of the semiconductor wafer such as the central area thereof, resulting in such a problem that the outer circumferential portion of the semiconductor wafer is polished to a smaller extent than the other area of the semiconductor wafer.
Consequently, the above conventional proposals to use the elastic pad and the diaphragm as the holding surface cannot apply a uniform pressing force to the entire surface of the semiconductor wafer to be polished.
It is therefore an object of the present invention to provide a workpiece carrier which is capable of applying a uniform pressing force to the entire surface of a workpiece such as a semiconductor wafer for thereby polishing the surface of the workpiece uniformly, and a polishing apparatus which incorporates such a workpiece carrier.
According to an aspect of the present invention, there is provided a workpiece carrier for holding a workpiece to be polishing and pressing the workpiece against a polishing surface on a polishing table, comprising: a top ring body for holding the workpiece; a retainer ring for holding an outer circumferential edge of the workpiece; a fluid chamber provided in the top ring body and covered by a resilient membrane, with a fluid being supplied into the fluid chamber; and a plurality of pressing members provided between the resilient membrane and the workpiece for pressing the workpiece against the polishing surface through the resilient membrane by a pressure of the fluid in the fluid chamber.
According to another aspect of the present invention, there is also provided a polishing apparatus for polishing a workpiece, comprising: a polishing table having a polishing surface thereon; and a workpiece carrier for holding the workpiece to be polishing and pressing the workpiece against the polishing surface. The workpiece carrier comprises: a top ring body for holding the workpiece; a retainer ring for holding an outer circumferential edge of the workpiece; a fluid chamber provided in the top ring body and covered by a resilient membrane, with a fluid being supplied into the fluid chamber; and a plurality of pressing members provided between the resilient membrane and the workpiece for pressing the workpiece against the polishing surface through the resilient membrane by a pressure of the fluid in the fluid chamber.
With the above arrangement, the fluid in the fluid chamber applies a pressing force to the pressing members, and the pressing members press the workpiece against the polishing surface on the polishing table through the resilient membrane. Since the pressing members impose a pressure in a continuous and uniform pressure distribution, the polishing pressure is uniformly applied to the entire surface of the workpiece for thereby uniformly polishing the entire surface of the workpiece.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.